This invention relates to a circuit arrangement for use in a radio receiver, such as a superheterodyne receiver, having a self diagnostic function.
A radio receiver of the type described generally includes a signal detecting circuit, for example, a squelch circuit which is for use in a radio communication system, such as a time division multiple access (T.D.M.A.) system. The circuit arrangement of this invention includes a part of the squelch circuit.
The radio receiver has an antenna at which various radio signals of the radio communication system arrive. The radio signals are delivered to a radio frequency (RF) band-pass filter (B.P.F.) which produces a desired RF signal. The desired RF signal is supplied to the circuit arrangement as a circuit input signal. The squelch circuit comprises a squelch signal producing circuit for detecting whether or not the desired RF signal is received. The squelch signal producing circuit produces a squelch signal when the circuit input signal or desired RF signal is weaker than a predetermined threshold level.
It is assumed that the radio receiver normally receives the desired signal. Under the circumstances, it is possible to judge whether or not the radio receiver itself is defective into a defective state by monitoring the squelch signal. The squelch signal is produced by measuring a reception field intensity of the radio signals in the signal detecting circuit, such as the squelch circuit.
Thus, the squelch circuit also produces the squelch signal not only when the radio receiver is put into the defective state but also when the field intensity of the radio signals becomes weak.
Accordingly, the above-mentioned radio receiver is difficult to distinguish between absence of the circuit input signal and the defective state. Therefore, the above-mentioned radio receiver can be used only when the desired signal is normally received.
In order to remove such a disadvantage of the above-mentioned radio receiver, it might be considered to provide a diagnosis input signal generated in the receiver when the circuit input signal is absent. Such a circuit arrangement with a self diagnostic function is selectively operable in a normal mode in response to the circuit input signal and a diagnosis mode in response to the diagnosis input signal. The circuit arrangement with the self diagnostic function comprises a controller for producing a control signal selectively indicative of the normal and the diagnosis modes. Responsive to the control signal, a processing arrangement processes the circuit and the diagnosis input signals into a processed signal of a processed level when the control signal indicates the normal and the diagnosis modes, respectively. Responsive to the processed signal and a reference signal of a reference level, the squelch signal producing circuit produces the squelch signal when the processed and the reference levels have a predetermined relationship. The reference level will presently become clear.
In this circuit arrangement of the above-mentioned type, the diagnosis input signal must have a level which is higher than the predetermined threshold level. Therefore, the circuit arrangement is disadvantageous in that the diagnosis input signal is radiated from the antenna as an interference wave to other receivers of the communication system.
A radio receiver of another type has active and backup stations coupled to each other through a coaxial relay. The radio receiver of the other type may have a circuit arrangement described heretobefore. In such a radio receiver, the diagnosis input signal leaks into the active station when the diagnostic input signal is supplied to the backup station in order to check the backup station. Therefore, the radio receiver of the other type is disadvantageous in that the diagnosis input signal interferes with the circuit input signal applied to the active station when the diagnosis input signal is used to check the backup station.